The field of the invention is functional magnetic resonance imaging (fMRI) techniques, and in particular, the use of fMRI images for surgical planning.
Functional magnetic resonance imaging (fMRI) technology provides a new approach to study neuronal activity. Conventional fMRI detects changes in cerebral blood volume, flow, and oxygenation that locally occur in association with increased neuronal activity induced by functional paradigms. As described in U.S. Pat. No. 5,603,322, an MRI system is used to acquire signals from the brain over a period of time. As the brain performs a task, these signals are modulated synchronously with task performance to reveal which regions of the brain are involved in performing the task. Much research has been done to find tasks which can be performed by patients, and which reveal in an fMRI image acquired at the same time, regions in the brain that function in response to the tasks.
Functional magnetic resonance imaging (fMRI) has been used extensively to study normal brain function, psychiatric conditions, learning disabilities, neurodegenerative conditions, recovery from stroke, and the relationship of eloquent cortex to brain tumors and arteriovenous malformations (AVMs). The pre-operative use of fMRI to identify eloquent cortex near resectable lesions is becoming a common clinical imaging scenario. Mapping eloquent cortex with fMRI relies on blood oxygen level dependent (BOLD) contrast. The physiological basis of BOLD signal is the regional vasoactive response induced by neuronal activity, causing increases in regional cerebral blood flow (rCBF), blood oxygen concentration, and consequently, fMRI signal. Yet, it is clear that BOLD contrast can be significantly compromised adjacent to regional cerebral pathology. For example, cortical BOLD signal can be reduced by the presence of glial tumors, both at the edge of the tumor and in vascular territories somewhat removed from the tumor. Loss of regional cerebral vasoactivity near lesions is thought to be a major contributing factor. When using BOLD fMRI, such effects may result in the underestimation of genuine neuronal function and may therefore cause an under-appreciation of the location of functioning cortical neurons near operable lesions. The result could be unexpected post-surgical neurological deficits.
Lesion-induced neurovascular uncoupling may also adversely affect assessments of cerebral dominance for certain key functions such as speech and language comprehension, movement control, and other cognitive abilities. It is common to assume that a greater extent and/or magnitude of cortical activation in one hemisphere of the brain are indicative of functional dominance in that hemisphere and that the opposite hemisphere may be subjected to more aggressive surgical resection with little risk. If, under pathological conditions, normal or near normal function is maintained but fMRI shows a shift in relative hemispheric activation away from the lesion, then it might be assumed that the function of the affected cortex has been taken over by the homologous area in the unimpaired hemisphere. Indeed, fMRI data showing a lesion-induced shift in relative cortical activation have been taken as evidence for cortical reorganization. However, this premise can be erroneous if, under certain conditions, the BOLD mechanism rather than neuronal function is selectively impaired.
Thus, the consequences of lesion-induced neurovascular uncoupling are to decrease the accuracy of BOLD fMRI information by underestimating genuine cortical neuronal function and by falsely implying cortical reorganization. The effects of this phenomenon on the process of pre-surgical planning and on the subsequent outcome of surgery are potentially catastrophic.